What's the point with that quite lackluster story in Endwalker?

[Kazhar]

I mean... they killed half the population to create Zodiark, then the half of the half to nurture life again, and then they were gonna kill EVERYONE new to bring back the old.

Is it really "killed half the population" when they did it in their own volition? You could say they were coerced into sacrificing themselves but nothing indicates this was true.
Also, no, they weren't going to kill everyone to bring back the old. The wording in never ambiguous about that, it's always "some of the new life created".

You're probably mistaking it with the Rejoinings, where the Ascians had the intention of killing all the Sundered in the end, but they were far gone at this point.

The whole takeaway for me was that these people (the ancients, all of them), as powerful as they were, they were as human and as flawed as anyone else. It's always been the most interesting thing about the Ancients for me. They're all different shades of Gray, Venat included.

This, I can agree. The Ancients were flawed and not perfect, what annoys me is that they're judged on completely different standards for their mistakes than the Sundered or our own mankind.
Hermes apparently didn't receive the amount of mental health care he needed? "Wow their society were a bunch of evil unfeeling fascists who deserved to die".
They had to make difficult choices to ensure the star would survive? "Their kind was doomed, it doesn't matter if Venat killed them."
Try to apply the same logic to humans and see how cruel this would be.

Unfortunately the story itself is guilty of this. Showing more compassion to the horrors done in Allag's name than the Ancients for commiting the crime of... not suffering enough in their daily lives I guess.